Optical imaging systems typically include a transmissive or a reflective imager, also referred to as a light valve or light valve array, which imposes an image on a light beam. Transmissive light valves are typically translucent and allow light to pass through. Reflective light valves, on the other hand, reflect only selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed behind the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid crystal display (LCD) projector configurations may become possible by the use of reflective liquid crystal microdisplays as the imager.
Many reflective LCD imagers rotate the polarization of incident light. In other words, polarized light is either reflected by the imager with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired grey scale. A 90° rotation provides the brightest state in these systems. Accordingly, a polarized light beam is generally used as the input beam for reflective LCD imagers. A desirable compact arrangement includes a folded light path between a polarizing beamsplitter (PBS) and the imager, wherein the illuminating beam and the projected image reflected from the imager share the same physical space between the PBS and the imager. The PBS separates the incoming light from the polarization-rotated image light. A single imager may be used for forming a monochromatic image or a color image. Multiple imagers are typically used for forming a color image, where the illuminating light is split into multiple beams of different color. An image is imposed on each of the beams individually, which are then recombined to form a full color image.
It is desirable to use as much light generated by the light source as possible. Where the light source generates light over a wide angle, such as an arc lamp, more light can be passed through the imager system using high f-number optics. A problem, termed “polarization cascade” and associated with a conventional PBS, places a lower limit on the f-number of the illumination optics of traditional optical imaging systems. A conventional PBS used in a projector system, sometimes referred to as a MacNeille polarizer, uses a stack of inorganic dielectric films placed at Brewster's angle. Light having s-polarization is reflected, while light in the p-polarization state is transmitted through the polarizer. However, wide angle performance is difficult to achieve using these polarizers, since the Brewster angle condition for a pair of materials is strictly met at only one angle of incidence. As the angle of incidence deviates from Brewster's angle, a spectrally non-uniform leak develops. This leak becomes especially severe as the angle of incidence on the film stack becomes more normal than Brewster's angle. Furthermore, there are contrast disadvantages for a folded light path projector associated with the use of p- and s-polarization.
Since light in a projection system is generally projected as a cone, most of the rays of light are not perfectly incident on the polarizer at Brewster's angle, resulting in depolarization of the light beam. The amount of depolarization increases as the system f-number decreases, and is magnified in subsequent reflections from color selective films, for example as might be found in a color-separating prism. It is recognized that the problem of depolarization cascade effectively limits the f-number of the projection system, thereby limiting the light throughput efficiency.
There remains the need for an optical imaging system that includes truly wide-angle, fast optical components that may allow viewing or display of high-contrast images with low optical aberration.